Other than the obvious, what's the difference between an Econo rigid letter (approximately .060″ thick) and the others (.080″ and .090″)?

The obvious being the “others” are thicker and usually, as you might expect, a little higher in price. The one thing an econo letter might do that the others won't is to occasionally, under certain wind conditions, move or “walk” sideways in front of another letter. They don't come out of the sign, and it's not a common occurrence, but just be aware that this can happen.